This invention relates to determining alignment or misalignment of a punch with respect to a die, in an apparatus in which workpieces are joined by self-piercing rivets, for example. One such apparatus is disclosed in U.S. Pat. No. 5,277,049 issued Jan. 11, 1994.
FIG. 1 shows an example of another self-piercing rivet setting apparatus in which the present invention may be employed. Typically, the apparatus 1 comprises a rigid C-shaped support frame 2, and a spindle unit 3 fixed to a first end (upper end in FIG. 1) of the C-shaped support frame. The spindle unit 3 holds an elongated tubular nosepiece 5 axially slidable through an end of the spindle unit (lower end in FIG. 1). A receiver 7 is fixed to the nosepiece 5 to receive self-piercing rivets supplied from a feed tube 6. A rod-shaped punch 9 is slidable axially in the nosepiece 5. A replaceable die 10 is seated at a position on the frame 2 facing the punch 9. The die has a portion centrally fitted in a bore 11.
Workpieces to be joined are placed between the punch and the die. A main shaft (not shown) axially movable in the spindle unit 3 is driven by a servomotor 13 and a reduction gear mechanism 14 to move the nosepiece 5 and the punch 9 toward the die 10. When the nosepiece 5 is brought into contact with a workpiece, the movement of the nosepiece toward the die is discontinued, but the punch 9 continues to move toward the die to press a self-piercing rivet, held axially at the end of the nosepiece, into the workpieces to be joined.
FIG. 2 shows the state after two workpieces 17 and 18, such as panels, are joined by a self-piercing rivet 15. As shown, the rivet includes a head 19 and a hollow smaller diameter leg 21 (typically of cylindrical shape) extending from the head toward the die 10. The rivet 15 is driven into the workpieces in such a manner that the leg 21 is expandingly deformed by the die while piercing the workpieces. The self-piercing rivet 15 penetrates the workpiece 17 located on the side of the punch 9 but stays in the workpiece 18 adjacent to the die 10 without penetrating that workpiece. Thus, no opening is formed in the exposed surface of the workpiece 18, preventing appearance degradation and blocking passage of noise and rainwater, for example. Panels typically joined in this manner are aluminum body panels employed in automobiles (e.g., to reduce weight) which are not easily welded.
If, during setting of a self-piercing rivet, as above described, the axis 23 of the punch and the rivet and the axis 25 of the die are aligned as shown in FIG. 2, a central protrusion 26 of the die 10 will be located at the center of the hollow leg 21, and the splayed portions of the leg will be symmetrically disposed with respect to the axis 25 of the die, whereby the workpieces 17, 18 are strongly connected. If, on the other hand, the axis 25 of the die 10 is misaligned with respect to the axis 23 of the punch and the rivet 15, as indicated by reference numeral 27 in FIG. 3, and if the misalignment is in excess of a tolerance, the expandingly deformed portion 22 of the leg 21 will not reliably join the workpieces 17, 18. More particularly, the tensile shear strength and peel strength of the workpieces will become unreliable. Furthermore, the leg 21 may protrude outside of the workpiece 18, forming an opening through which rainwater, for example, may pass, which may cause a corrosion problem.
Because of the manner in which each self-piercing rivet 15 is held in the nosepiece 5, there is little likelihood of misalignment between the punch 9 and the rivet. However, the die 10, located at a position away from the nosepiece 5, may become axially misaligned with respect to the punch and a rivet. The manufacturer of the riveting apparatus will take appropriate steps to avoid such misalignment. However, occurrences at a user's job site may cause misalignment beyond the control of the manufacturer. Resulting defective riveted joints may not be easily detected by the user of the riveting apparatus. Moreover, returning the riveting apparatus to the manufacturer to check alignment is burdensome and may, undesirably, require use of standby apparatus.